Gravitational waves

by Wm. Robert Johnston
last updated 24 September 2001

Gravitational waves (GWs) are ripples in space-time and can come from sources like black holes and/or neutron stars spiraling into each other, from the formation of black holes, from rotation of neutron stars, and other things.

GWs produce slight (for us) distortions in space. They are hard to detect because they don't interact with matter. The calculated effect of GWs on Earth from sources like those in the above is stretching and compressing of space by about one part in one trillion billion. Click here for illustrations of this.

General relativity tells us that an object with mass will curve space-time around it; we refer to this space-time curvature as its gravitational field. Any change in this gravitational field can radiate only at the speed of light. Thus the rapid acceleration of the object will produce ripples in space-time curvature that radiate outward at the speed of light--gravitational waves.

So how do we know they exist? A pair of neutron stars was discovered in 1974 and are observed to be spiraling in towards each other. This means they are losing energy, and the amount of loss is exactly what Einstein's theory predicts they would lose to GWs. For more information on this binary neutron star system, click here.

To get a sense of what gravitational wave observers are trying to do, consider the following analogy:

Imagine being in a large room filled with people conversing with one another-and you are deaf. You can look around, see people, study them and reach a variety of conclusions. But you can't tell what people are saying to each other (assuming you can't read lips). Also, it's hard to see people far away from you.

However, if you can hear then a whole new realm of information is available to you. Vision and hearing each provide completely different types of information. You can now start evaluating conclusions you drew from observing people.

Now that you can hear conversations from all over the room, you may actually have a problem. With vision, you had no problem directing your attention to a single person. In contrast, when you listen you hear any and all noises in the room at the same time, superimposed on each other.

If people are rarely talking, with perhaps only one conversation occurring at a time, there is little difficulty following a single conversation. If many conversations are occurring simultaneously, you have a problem. If one conversation near you is particularly loud, you can make it out over the rest. It also helps if you can concentrate on the voice of a single person. If you can single out individual conversations one way or another, you will likely hear them from further than you can see-perhaps even from near the edges of the room.

Another problem is determining where a conversation is coming from. If you can only hear with one ear, this is hopeless. If you can hear with both ears, however, your brain interprets the time delay between sound reaching each ear and you can tell what direction the conversation came from. You may even be able to match that up with a specific person or persons, giving more insight.

Until you actually start to hear for the first time, you can't know what to expect. Based on your observations by sight, you may have some idea what to expect. But you don't know how loud or soft the conversations are. You don't know if you will be able to understand individual conversations over the background. You also may find completely unexpected sounds-for example, perhaps there is background music playing.

How is all this analogous to gravitational wave astronomy? Gravitational wave astronomy is an entirely different way of looking at the universe than previous electromagnetic radiation observations (various forms of light), so it will provide information we have not had before. Gravitational wave detectors pick up waves from most directions simultaneously, so isolating a single source will require techniques for seperating a given signal from all the other noise. Determining the direction of a source will require multiple detectors. If there is much background noise, the task will be challenging. But there is also the prospect of learning things we had no inkling of from out electromagnetic observations of the universe.

Now this analogy is limited in several ways, including the following: In the hypothetical room, you can't see people very far away because other people obstruct your view. In the universe, matter is spread out enough that our view is not blocked so much, but light from very distant objects is too dim to detect. Light (and sound) energy falls off in proportion to the square of the distance to the source, but the detectable effect of gravitational waves falls off slower, in proportion to the distance.

The reason for this is that in general the energy per unit area carried by waves is proportional to the inverse square of the distance of the source, whereas the wave amplitude is inversely proportional to the distance. For example, if you increase your distance from a light source by a factor of three then your eyes will receive only one-ninth as much light energy. For essentially all means of detecting electromagnetic radiation, energy is what is measured. This means that you must quadruple the sensitivity to double the distance at which you can detect a given object.

All gravitational wave detectors built or proposed to date are directly sensitive to the amplitude of GWs, not the energy. Thus, quadrupling their sensitivity would allow them to detect a given source at a distance four times greater than before. A consequence is that GW detectors that have been proposed could routinely detect events billions of light years away.

A further illustration: the normal human eye is quite easily able to see Jupiter in the night sky. It can barely detect one galaxy at a distance of two million light years. At the same time, the collisions of neutron stars in galaxies billions of light years away produce GWs which carry even greater amounts of energy through the same eye. Since these waves do not interact with matter, however, they go unnoticed.

Detectable GWs require very massive objects and very rapid motion. The best candidate sources, black holes and neutron stars, are difficult to impossible to directly observe by other means. Gravitational wave detectors may thus provide information on these objects never available before.

Potential sources of gravitational waves can be grouped into three categories: burst sources, periodic sources, or stochastic sources. The following list summarizes some expected or possible sources (in each group listed roughly by descending expected detectability):

Burst sources (signals lasting from a fraction of a second to about 20 seconds):